1. Field of the Invention
The present invention relates generally to improvements in a commercial process for non-tubular cracking of heavy hydrocarbon feeds for the purposes of producing ethylene and other valuable olefin products as well as chemical grade synthesis gas.
2. Prior Art
A great deal of prior work has been done in connection with the cracking of hydrocarbon feed stocks to obtain basic chemicals such as ethylene, acetylene and propylene. Presently, the most common approach involves the cracking of a hydrocarbon feed in the presence of steam in a fired tubular furnace. In such a steam cracking process the thermal energy of the combustion gases is transferred to the feed through the metal walls of the tubes and therefore the tube metallurgy becomes one of the limiting factors with respect to the maximum cracking temperature which is usually several hundred degrees below that which can be achieved in a non-tubular cracking process such as the diacritic cracking process described herein. To obtain proper cracking conditions, the residence time in a steam cracker must be substantially longer to compensate for the lower temperatures. For example, residence time of about 0.25 to 0.50 seconds is typical in many modern steam cracker designs. Such longer residence times lead to a furnace effluent hydrocarbon composition that is significantly different from that obtained from a process utilizing a non-tubular reactor. For example, acetylene and ethylene yields are generally lower but the yields of propylene, C.sub.4 olifins and pyrolysis gasoline are usually somewhat higher.
In view of the shortcomings of steam cracking, considerable work has been done in connection with the thermal cracking of hydrocarbons to obtain higher yields of ethylene or in some instances, acetylene. For example, U.S. Pat. No. 2,790,838 deals with the general process of a single pass cracking method for the production of mixtures of acetylene and ethylene (with the primary emphasis on acetylene) in which a hydrocarbon feed stock is cracked by "thermal shock" when contacted with hot gases produced by the combustion of a fuel. This patent does not teach a process operating at elevated pressures and does not discuss the elimination of coking problems which result in such a process, especially when having hydrocarbon feed stocks are used, such as residual oils, crude oils, vacuum gas oils atmospheric gas oils and coal derived liquids.
Other prior art patents are known which deal with thermal cracking of certain hydrocarbons. U.S. Pat. No. 3,320,154 discusses the use of combustion gases which are mixed with naphtha and steam at elevated pressure (about 90 psia). The cracked gases are adiabatically expanded in a turbine to drive the turbine while rapidly cooling the gas. The latter patent teaches the use of naphtha and does not deal with heavy feeds since it is believed that heavy unreacted feeds and resulting products would foul the turbine.
U.S. Pat. No. 2,751,334 involves a process to vaporize hydrocarbons and produce coke from a hydrocarbon feed. Some olefins are produced but in relatively low yields. A feed is sprayed into a vortex of hot gases. The hot gases may be preheated by air or an oxygen containing gas which combusts part of the feed. Pressure is employed in the reaction zone but is not considered critical since both atmospheric subatmospheric or super-atmospheric pressures can be used. The preferred pressure is atmospheric pressure or slightly elevated pressure such as 5 to 30 psia. The overall emphasis of the invention was the production of a vaporous hydrocarbon fraction and a particulate coke fraction.
U.S. Pat. No. 2,912,475 teaches a process in which ethylene and acetylene are manufactured by contacting a hydrocarbon with a carrier gas consisting of a hot combustion gas and a secondary gas. The secondary gas has the same chemical nature as the combustion gas but contains hydrogen and functions to remove the oxygen molecules and atoms from the combined gas stream before pyrolysis. The latter process appears to be a somewhat more complicated version of a steam cracking process.
U.S. Pat. No. 3,959,401 relates to a reactor design for cracking hydrocarbons by mixing with hot gases. Tangential feed flow is used in the reactore. The hot gases may be steam or combustion products. The reactor of the latter patent does not deal with the prevention of coking or the utilization of gas filming or any other technique to minimize coking of the reactor.
U.S. Pat. No. 3,178,488 describes a process for thermally cracking a low boiling paraffinic hydrocarbons in a flame cracking process. The linear velocity of the hydrocarbon feed must be controlled (e.g., 50 to 250 feet per second) in a 7 to 11 inch diameter Venturi throat. The process involves two cracking zones which are formed within a reaction zone. One is an interior cracking zone and the other an exterior cracking zone which is generally annular in relation to the interior zone. The interior zone is characterized by a high velocity gas, high temperature and thorough mixing of the combustion gases with the hydrocarbon cracking feed. The exterior cracking zone is characterized by lower velocity, lower temperature and by the fact that the hydrocarbon feed is not so thoroughly mixed with the combustion gases. This process results in the simultaneous production in the entire reaction zone of acetylene, ethylene and propylene. Also, one of the main aspects of the latter process is the incomplete or non-uniform mixing of the reacted hydrocarbon feed with the hot combustion gases in the Venturi section and in the reaction completion zone.
U.S. Pat. No. 2,823,243 involves a process in which hydrogen and oxygen are burned and mixed with a gaseous hydrocarbon stream. The flow velocity is increased, then decreased, then increased after quenching and an abrupt change in direction. A helically moving blanket of tempering gas is disposed about the combustion gas to cool the combustion gas from about 4500.degree. F. to 5300.degree. F. to not higher than 4200.degree. F. No mention is made of elevated pressure and only gaseous hydrocarbons are treated.
Some prior art patents dealing with the thermal cracking of feeds require very high velocities (e.g. sonic or near sonic velocities) in the cracking reactor. For example, U.S. Pat. No. 2,767,233 discloses a process in which combustion gases are mixed with an aliphatic hydrocarbon feed. High velocities of at least 1,000 feet per second are taught. Also, U.S. Pat. No. 3,408,417 involves the use of sonic velocities in the feed injector and in the reactor. The diacritic cracking of the present process is achieved with substantially lower velocities in the cracking reactor stage (e.g. 250-350 feet per second) and at elevated pressure of about 70 psia to 1,000 psia. The control of the present cracking process is easier and the cracking is more selective.
In addition to the above-mentioned patents, the Applicants are also aware of other patents which deal generally with processes or apparatus which involve thermal cracking of hydrocarbon streams or related technology for obtaining acetylene, ethylene or other olefin type products. The patents of which Applicants are aware include U.S. Pat. Nos. 2,985,698, 2,934,410; 3,301,914; and 3,579,601. Applicant is also aware of U.S. Pat. No. 4,035,137 which deals with a fuel burner for the combustion of liquid and gaseous fuel which has been designed to minimize the levels of nitric oxides produced. The fuel burner of the latter mentioned patent also includes inlet orifices for introducing a stream of air into the burner for purposes of preventing coking.
In addition to the above-mentioned patents, the Applicants are also aware of certain experimental work which has been conducted in connection with a process for the thermal cracking of a hydrocarbon (i.e., hexane) into ethylene, acetylene and other by-products such as propylene and butadiene. Such process was of an experimental nature to determine yields and feasibility, and therefore involved only short-term testing which did not consider the prevention of coking, which is a severe practical problem in commercial installations. There were no "decoking" techniques or steps in the combustor, reactor or heat exchange sections of such experimental process. Also, the experimental process used hexane as a feed stock and accordingly the process and related equipment were not designed to handle and inject heavier and more troublesme hydrocarbon feed stocks such as those of the present invention.
None of the prior art processes, which Applicants are aware of, present a practical and economical process for producing high yields of ethylene (along with other valuable olefin products and synthesis gase) by the diacritic cracking of heavy hydrocarbon feed stocks (e.g., residual oils, crudes, coal liquids, etc.) at elevated pressures (i.e., about 70 psia to 1,000 psia) in such a way that the reactor and subsequent heat exchange equipment will not be adversely affected by severe coking. The main advantages of the invented process over that of the known prior art will be apparent from the detailed description of the invented process described hereinafter.
Until quite recently, there has not been much emphasis on the use of hard to handle fuels, such as resids and crudes. The economics were such that feed stocks such as naphtha were readily available and inexpensive. With diminishing supplies and higher proces for lighter and easier to handle feeds, it is desirable to use the heavy hydrocarbon feed stocks which are not in great demand and which are considerably less expensive. The present process allows for the economical use of such heavier feed stocks by selective diacritic cracking which optimizes the ethylene yield, produces valuable chemical grade synthesis gas and also eliminates the coking problems which in the past have severly restricted the use of resids, crudes and other heavier feedstocks in continuous thermal cracking processes. Conventional steam cracking processes have been generally confined to lighter hydrocarbon feeds such as naphtha.